The invention relates generally to data communications and more particularly to high speed data transmissions over the public switched telephone network.
The sudden popularity of the Internet as a communication tool has led to an intense push for higher data transmission rates over the Public Switched Telephone Network (PSTN). As a result, the demand for increased data transmission rates over analog twisted pair is at an all time high. The most recent widespread standard is xe2x80x9c56Kxe2x80x9d analog modem technology developed by U.S. Robotics and Rockwell/Lucent. While these technologies will not generate true 56 kbps performance under typical subscriber line conditions, they do provide a boost in performance from the previous standard of bidirectional 33.6 kbps.
Theoretically, a connection of 64 kbps should be attainable between the subscriber and the Internet Service Provider (ISP) via a standard Plain Old Telephone Service (POTS) connection. This is because 64 kbps is the rate at which data is transferred from the Central Office (CO) linecard to the ISP or other remote terminal. Several factors prevent this from happening including imperfect line conditions and varying local loop lengths common to POTS analog networks. The primary reason, however, for this less than the theoretical transmission rate is that the PSTN was designed to carry voiceband frequencies in the range of 300-3.4 kHz.
With the advent of digital voice systems, the decision was made to use a xe2x80x9ccompandedxe2x80x9d (compressed/expanded) data to reduce the number of bits per digital sample from a nominal 13-bits to a companded 8-bits. Companding schemes use higher resolution at low signal amplitudes and lower resolution at high amplitudes. Companded signals are suitable for voice frequencies but not for analog modems since they limit their apparent bandwidth to a ceiling of 33.6 kbps upstream and 56 kbps downstream. Thus, companded code is a poor carrier for data signals.
Moreover, while the use of 56K standards results in downstream data throughput of 56 kbps under ideal local loop conditions, the upstream direction must still contend with an A/D conversion into 8-bit companded data and is still limited to 33.6 kbps. Imperfect conditions in the analog local loop further degradate the signal resulting in less than the 56/33.6 kbps maximums. If higher data throughput is to be achieved, the limitations in the CO need to be overcome. Overhauling the PSTN by replacing the 8-bit companded data scheme could solve the problem, but this is not a feasible solution since the cost of such as effort would be enormous.
The invention overcomes limitations in bandwidth of prior communications standards including 56K by offering increased downstream rates using an analog modem communicating over the PSTN.
Disclosed in one embodiment, is a linecard codec for communicating data over a Public Switched Telephone Network (PSTN). Subscribers and services providers communicate through a central office facility. The service providers are coupled to the central office via a digital backplane. The codec comprises an analog interface to the PSTN with a signal converter coupled to the analog interface and configured to convert analog signals from subscribers to equivalent digital linear data. A mapping means is coupled to the converter and arranged to transform linear data to any other mapping scheme for transmission on the backplane. Transmission occurs through a digital interface coupling the codec to the digital backplane.
In one embodiment, the mapping means comprises a RAM table that stores mapping values. The mapping values determine the coding scheme between the data transmitted by a service provider on the digital backplane and the linear code transmitted to a subscriber. In this manner the limitations of companded code are overcome.
According to another embodiment, the linecard incorporates an output register that receives the output of the RAM table and converts it to PCM output signals which are transmitted on the digital backplane to the service provider.
According to another embodiment, the RAM table is configured to store mapping values transferred to the codec directly from the service provider. In the absence of a specific data map, the linecard defaults to linear mapping.
Also disclosed is a communications network wherein the data rate is proportional to a predetermined mapping function provided by a central office facility. The network comprises at least one subscriber having an analog interface to a PSTN, the subscriber capable of generating analog signal waveforms for transmission on the PSTN via the analog interface. A link forms a communications channel between a central office facility and the subscriber. At least one service provider communicates is coupled to the central office facility through a digital backplane. The central office includes a linecard codec configured to receive analog signals from the subscriber and transforms them to equivalent digital bit stream sequences. A mapping function in the linecard codec is provided to adjust the transmission rate of the sequences to be proportional to a predetermined companded scheme stored in the codec.
A technical advantage of the invention is that it provides the subscriber with much more bandwidth than is currently available since it overcomes the limitations of companded data. Thus, the full bandwidth of the digital backplane can be used increasing the overall data rate of the communications network.
Yet another technical advantage of the invention is that it permits replacement of the existing linecard in the CO with the linecard of the present invention enabling hardware and software changes at the CO to provide the increased bandwidth. This eliminates the need to employ a direct customer interface with the CO and thus no equipment needs to be installed at the subscriber""s residence.